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RF interference issue again (esp. for E Drucker and Jim Weir and other RF wizards)



 
 
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  #1  
Old November 5th 03, 03:21 AM
Snowbird
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Aaron Coolidge wrote in message ...

Sydney, have a look at the fcc link I posted a couple messages ago. It lets
you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.


Thanks.

: Do other aircraft report the same interference?


: Not that I've heard, but then, I might not have heard.
: Or, like us, they might have assumed it was a problem in
: their airplane.


Update: I talked to a local DE who is also doing piles of
instrument instruction. She says she's flying in beaucoup
planes in that area, without the same problem.

So it *is* something specific to our plane I guess.
Although it's an intermittant problem for us, too.

: We didn't have this problem before last spring.


Did channel 2 recently add a digital TV transmitter? Like, last spring?


I believe so, yes.

So here's what I'm thinking.

That tower is TV Channel 2 (60-65 MHz I think?)
Channel 5 which is nearby would be 79-84 MHz.

This makes me think that marker beacons, at 75 MHz,
are the logical suspect for causing a problem.

But can the marker beacon antenna, by itself, be somehow
bringing signals into the plane to be received by the
rubber whip antenna of our handheld?

If this is a possibility, how do we safely remove the
marker beacon antenna for testing purposes? Do we need
to put some kind of load on the cable heading for the
marker beacon receiver, since we can't turn the MB off
while the power in the plane is on?

If removing the MB antenna seems to cure the problem,
what do we test or do? Is the antenna itself likely to
be bad and in need of replacement, or is this likely to
be a ground type issue where maybe we should replace the
coax, or at least redo the connections?

If it seems far-fetched that the MB antenna itself is
the culprit, where next do we look?

Thanks,
Sydney
  #2  
Old November 5th 03, 05:21 AM
Aaron Coolidge
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: Sydney, have a look at the fcc link I posted a couple messages ago. It lets
: you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.

: Thanks.

Cool, I thought that you might have kill-filtered me for some reason, because
no one other than me seemed to be getting my messages!

: So it *is* something specific to our plane I guess.
: Although it's an intermittant problem for us, too.

Aha, I think we're on to something.

: : We didn't have this problem before last spring.
:
: Did channel 2 recently add a digital TV transmitter? Like, last spring?

: I believe so, yes.

On further reflection, this may be a red herring. Digital TV is in the
220+ MHz region.

: So here's what I'm thinking.

: That tower is TV Channel 2 (60-65 MHz I think?)
: Channel 5 which is nearby would be 79-84 MHz.

: This makes me think that marker beacons, at 75 MHz,
: are the logical suspect for causing a problem.

Channel 2 is 54 to 60 MHz, the 2nd harmonics of are 108 to 120 MHz.

This leads me to suspect one of the *NAV* radios. Can you physically
remove them from your plane, one at a time, and leave them in your car?
This would take their front end circuitry out of the area. Then try the
other one. The COM radios would also be out of the picture. Perhaps
you've already done this?

The reason that I'm harping on radios is that intermodulation
distortion needs a detector or a modulator to occur, such as in the
RF front end of a radio. I don't think that an antenna by itself is
sufficient to cause it.

Also, you might try taking out the nav antenna splitter. I'm not sure
that this should make any change, but if we're using buckshot methods...

Since I changed jobs I don't have my trusty HP 8591E spectrum analyzer
anymore, if I did I'd consider a trip to St Louis!

: But can the marker beacon antenna, by itself, be somehow
: bringing signals into the plane to be received by the
: rubber whip antenna of our handheld?

If you disconnect the MB antenna from the MB receiver, it is unlikely
that the end of the coax could act as much of a radiating element. I have
made a passive radiator before, but that's 2 antennas connected to each
other.

: If this is a possibility, how do we safely remove the
: marker beacon antenna for testing purposes? Do we need

If you've got the bent metal rod kind, disconnecting the little floating
wire will disconnect the MB antenna from the in-plane electronics, though
I'm inclined to dismiss the MB system.

If you wish to electrically remove the antenna from the plane while leaving
it physically in place, you can get a "terminator" cap from most electronics
stores that cater to the ham radio crowd. I'm not sure Radio Shack sells them.
You'd want a 50-ohm terminator, and whatever adapters are needed to connect
it to the end of the antenna coax. If I were doing this, I'd probably
terminate the RF input to the radio, as well.

Please keep us (me) informed, we're trying to help the best we can!
--
Aaron Coolidge (N9376J)
  #3  
Old November 5th 03, 12:40 PM
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In article ,
Snowbird wrote:


Aaron Coolidge wrote in message
...

Sydney, have a look at the fcc link I posted a couple messages ago. It lets
you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.


Thanks.

: Do other aircraft report the same interference?


: Not that I've heard, but then, I might not have heard.
: Or, like us, they might have assumed it was a problem in
: their airplane.


Update: I talked to a local DE who is also doing piles of
instrument instruction. She says she's flying in beaucoup
planes in that area, without the same problem.

So it *is* something specific to our plane I guess.


"Not necessarily", although it _is_ likely. Different radios _do_ have
different degrees of susceptability to intermod interference. It _is_
possible that you have 'unlucky' radios.

I repeat: "possible", yes; "probable", no.

Although it's an intermittant problem for us, too.


The _definitive_ test -- to confirm that it *is* something with
your plane -- is to get her, in one of her planes that does *not*
exhibit the problem, to fly in the area, *WITH*YOUR*HAND-HELD*,
at a time when you _are_ able to reproduce the problem on *your* radios.

If the hand-held does _not_ misbehave, then it _is_ confirmed to be
something in your plane.

There is _also_ a REMOTE possibility that, *because*the*hand-held*is*aboard*,
The in-board radios _will_ hear interference. This would be conclusive
proof of 'front end overload' in the hand-held, with it _re-radiating_ the
spurious signal, which is then being picked up by _their_ recievers.


: We didn't have this problem before last spring.


Did channel 2 recently add a digital TV transmitter? Like, last spring?


I believe so, yes.

So here's what I'm thinking.

That tower is TV Channel 2 (60-65 MHz I think?)


54-60 mHz carrier at 55.125

Channel 5 which is nearby would be 79-84 MHz.


76-82 mHz carrier at 77.125

This makes me think that marker beacons, at 75 MHz,
are the logical suspect for causing a problem.


No particular reason to suspect the marker beacons.

'intermodulation interference' occurs when the frequencies of
two (or more) transmitters add/subtract to give a result that
is the same as the "real signal" you're looking for. Frequently,
the 'source' signals are _far_removed_ from the frequency that
is getting fouled. e.g. a Tx at 400.0mhz, and a 2nd one at 526.50mhz,
combining to give a spurios signal at 126.50 mHz. (note: I'm pulling
figures out of thin air here, *no* reason to believe there's anything
at 400.0, or 526.5, in your area.

But can the marker beacon antenna, by itself, be somehow
bringing signals into the plane to be received by the
rubber whip antenna of our handheld?


yes, and no. grin

*Any* place where two _dis-similar_ pieces of metal come into contact
is a "low grade" transistor junction, providing a place where 2 or more
signals can "mix", generating a 'spurious' hetrodyne, or intermodulation
product. This "generated" (low strength) signal can then 'radiate' from
_any_ antenna-like piece of metal that is electrically connected to the
point where the mixing occured.

If this is a possibility, how do we safely remove the
marker beacon antenna for testing purposes? Do we need
to put some kind of load on the cable heading for the
marker beacon receiver, since we can't turn the MB off
while the power in the plane is on?

If removing the MB antenna seems to cure the problem,
what do we test or do? Is the antenna itself likely to
be bad and in need of replacement, or is this likely to
be a ground type issue where maybe we should replace the
coax, or at least redo the connections?

If it seems far-fetched that the MB antenna itself is
the culprit, where next do we look?


There are _still_ two possibilities to deal with -- 1) it _is_
something inside the plane, 2) it is *not* something inside
the plane.

The fact that you've got a hand-held that will 'hear' the problem
is a good start at a 'signal sniffer'.

A "sniffer" needs two capabilities that aren't 'standard' on the
hand-held. One, a way to reduce the incoming signal strength to
a point where you can 'hear' or 'see' (on an s-meter, if present),
comparatively small changes in signal strength. And, two, a
'directional' antenna.


Assuming the hand-held has a -removable- antenna (probably the
so-called "rubber duckie" type), this is all relatively _easy_ to
do.

Step 1 is to build a 'variable attenuator'. The ideal enclosure to
build this in is a small "U-box" (available from radio shack, among
other places -- all metal, 2 pieces, each shaped sort-of like the
letter "U"). several minature DPDT switches, each of which switches
an attenuator stage (of varying degree). the attenuator stages
consist of a series resistor, and a resistor to ground, so as to
provide a constant impedence. Eached switched stage is wired in
series to the next switch, so you can "add" attenuation, just by
switching in additional stages. Without having _any_ idea of
signal levels, I'd suggest 5 stages -- one at 3db, one at 6db,
one at 10db, and 2 at 20db ea. This lets you cut signal strength
in 3db increments (cutting the signal seen by the reciever in half)
all the way to nearly 70db of attenuation (enough to drop out a
fairly high-powered source at close range.

Step 2, a 'directional antenna' is required. It doesn't have to be
an 'efficient' antenna, just 'directional'. In fact, 'efficieny'
in _this_ application is *not* a good thing. A simple one can be
manufactured with nothing more than: (a) a piece of co-ax, (b) a
short board to serve as a 'handle' (something like a 12" piece of
1x2 is near-ideal), and a 'medium' (circa 6" across the top) _metallic_
funnel. Drill a hole through the board, a couple of inches down from
one end. big enough that the spout of the funnel just fits in it.
Take the piece of coax, and strip back the outer cover, and shielding,
about 2/3 of the distance from the top of the funnel to the tip.
strip off the inner dielectric as well, but _leaving_ a section that
is roughly the length of the spout. "Apply" the coax to the funnel,
from the spout end, so that you have the bare center wire sticking up
in the middle of the funnel. Solder the shield to the tip of the spout.
Lastly, fit this 'monstrosity' back into the hole in the wooden 'handle'.

Now, by simply 'waving the stick around', you can point the funnel in
any direction you choose.

Run this 'antenna cable' to the attenuator box, and cable from the
box to the antenna connection on the radio.

Ok, you're equipped for 'transmitter hunting'. *grin*

A little experimenting with a "known" station -- say a 'weather' announce
loop, will show how the beastie works. Starting with all the attenuation
'switched out', hold the stick upright, and rotate it 360 deg. If you
hear the station at all angles, start switching in some attenuation. You'll
get to a point where you only hear the station when the funnel is pointed
"more or less" in it's direction. The station is located roughly in the
middle of that arc where you can hear it.

*NOW* you're ready to see what's to be seen about the source of your
interference.

Fly into the problem area, and tune to the interference. and locate
what direction it's coming from. Now, make another pass through the
area, on a course 90 degree removed from the first attempt.

When you localize the direction _this_ time, one of two things will occur.
either the _absolute_ direction will be the same (e.g "due East"), or
it will be the same _relative_bearing_ (e.g. 45 degrees left of 'straight
ahead'). If it's the first, the problem is *outside* the plane. If
the second, it _is_ in the plane.

If the source is outside the plane, you're conclusively dealing with front
end overload intermodulation interference. There are two possible solutions:
1) high-Q bandpass filtering, to eliminate the out-of-band interference
sources.
2) replace the radios, with ones that aren't as suceptable to interference.

If the source is in the plane, you keep hunting. Do the direction check from
different places in the plane. Remember to check on all axis (roll,pitch,
yaw) too. (maximize on one axis, then hunt 90 degree _relative_ to that one,
and confirm by using 3rd 90) By kicking in additional attenuation, you should
be able to get fairly narrow arcs where you can hear the interference, and
tell "something" about where problem is located. if it shows 'straight ahead'
when checked near the left window, but 45 deg. left of straight ahead
when checked from the right window, this tells you a _lot_ about where
to look. Admittedly, if the problem is 'way back in the tail', you're
not going to get a very precise indication. Unless you get "Mini-Me"
to go back there with the antenna stick, that is. guffaw



  #4  
Old November 5th 03, 01:21 PM
Paul Sengupta
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Bear in mind that the radios involved may have different front
ends. It might just be a coincidence that your handheld and
panel radios are affected and nothing to do with the airframe.

How about 127.00-10.7 (IF mixer) = 116.30
116.30/2 = 58.15?
(i.e. mixing of the second harmonic of 58.15MHz)

Ok, unlikely, just playing with numbers!

Paul
G1YJY

"Snowbird" wrote in message
om...
Update: I talked to a local DE who is also doing piles of
instrument instruction. She says she's flying in beaucoup
planes in that area, without the same problem.



  #5  
Old November 7th 03, 11:08 PM
Gene Seibel
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I operate out of St Louis Regional and have noise on 126.00 on a KX170B.
--
Gene Seibel
Hangar 131 - http://www.pad39a.com/gene/plane.html
Because I fly, I envy no one.


Do other aircraft report the same interference?


Not that I've heard, but then, I might not have heard.
Or, like us, they might have assumed it was a problem in
their airplane.

We didn't have this problem before last spring.

Any assistance sorting this out would be greatly appreciated;
we do have a pretty good local avionics guy but he frankly
seems stumped (at least he's honest and good enough not to
simply suggest replacing all the radios in the stack, which
one shop did).

Cheers,
Sydney

  #6  
Old November 4th 03, 06:31 AM
Aaron Coolidge
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In rec.aviation.owning Jim Weir wrote:
: Quick question(s) ...

: Is TV channel 3 a local station for you? Is their transmitter in this antenna
: farm? Do other aircraft report the same interference?

: Jim

Jim, do you think that TV Channel 2 (54 MHz) could be interfering with the
NAV radios? I do know that I can receive distorted FM on my nav radio
at 108.00 (we have a 107.9 FM station in town here).

--
Aaron Coolidge
  #7  
Old November 3rd 03, 11:39 PM
Jim Stockton
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Snowbird wrote:

OK, we're still having our RF interference problem and
our avionics guy pleads 'stumped'. Meanwhile we're going
nuts whenever we need to get radar vectors for the ILS at
our local Class D or when we depart IFR to the SE.

Here is what we know

1) the problem is intermittant. occurs both at night and during day.
2) when it does occur, the problem occurs in a specific
area -- heading towards a local antenna farm
3) legitimate radio transmissions come through loud and clear
4) the interference isn't just random noise, but sometimes has
voices in it (like a radio or TV show)
5) we have disconnected the ELT from its antenna (but left
it turned off in the back seat of the plane) -- problem persists
6) marker beacons on, marker beacons off, nav radios different freqs,
nav radios off, no effect
7) swapped our KMA 20 audio panel for a loaner KMA 20 no difference
8) we have tried turning off the airplane's entire electrical
system and listening for interference on a handheld radio with its
own "stick" antenna. Problem persists (!!!!)
9) we have tried different frequencies while experiencing the
interference -- not exhaustively. here is a list (- means no
interference + means interference)

124.00 -
124.20 -
124.52 -
125.00 -
126.00 +
126.50 +
126.50 mb on, mb off, nav 111.9, nav 110.8, nav off
126.50 handheld w/ alt off, airplane electrical system off
127.00 +
127.10 -
127.25 -
127.27 +
127.30 -
127.50 +
127.97 -
128.00 -
129.00 +
130.00 -
131.00 -
132.00 +

(126.5 is the local tracon frequency where the interference is
problematic for us, which is why I focused there. 127.0 might
be the strongest interference)

geographical location where interference seems strongest
(there's an antenna there, and when we were directly over
it interference stopped)
38 31 90
90 21 75

Can we figure out the frequency and maybe the station which
is causing the problem from the above info?

Ideas? Other tests? Things to check? Help! If we still
get the problem with the plane's entire electrical system off
and using a radio/antenna which is not connected to the plane,
is there ANYTHING we can do or must we just grit our teeth and
bear this?

Plane's equipment:
Sigtronics SCI-4 intercom
KMA20 audio panel/mb
King KI-170B nav/com
TKM 170B nav/com
Apollo 2001 IFR GPS
King KN-75 glideslope receiver
King KT-76 Transponder
no ADF or DME

THANKS!
Sydney
Grumman AA5B "Tigger"


It sounds like IMD in the receiver frontend. As Jim pointed out it is
probably being caused by the input amp stage. The difficult thing is the
cure. I don't know if a high power band pass filter is available so that
the interfering frequencys don't get into the reciever front end. If you
can pinpoint the source frequency of the interferance a simple coax
tuned stub filter will notch it out but that would not be broad enough
for a TV transmitter, but usually only for an AM transmitter, or other
narrow band interferance.
Good Luck
Jim Stockton
  #8  
Old November 4th 03, 02:18 AM
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In article ,
Snowbird wrote:


OK, we're still having our RF interference problem and
our avionics guy pleads 'stumped'. Meanwhile we're going
nuts whenever we need to get radar vectors for the ILS at
our local Class D or when we depart IFR to the SE.

Here is what we know

1) the problem is intermittant. occurs both at night and during day.
2) when it does occur, the problem occurs in a specific
area -- heading towards a local antenna farm
3) legitimate radio transmissions come through loud and clear
4) the interference isn't just random noise, but sometimes has
voices in it (like a radio or TV show)
5) we have disconnected the ELT from its antenna (but left
it turned off in the back seat of the plane) -- problem persists
6) marker beacons on, marker beacons off, nav radios different freqs,
nav radios off, no effect
7) swapped our KMA 20 audio panel for a loaner KMA 20 no difference
8) we have tried turning off the airplane's entire electrical
system and listening for interference on a handheld radio with its
own "stick" antenna. Problem persists (!!!!)
9) we have tried different frequencies while experiencing the
interference -- not exhaustively. here is a list (- means no
interference + means interference)

124.00 -
124.20 -
124.52 -
125.00 -
126.00 +
126.50 +
126.50 mb on, mb off, nav 111.9, nav 110.8, nav off
126.50 handheld w/ alt off, airplane electrical system off
127.00 +
127.10 -
127.25 -
127.27 +
127.30 -
127.50 +
127.97 -
128.00 -
129.00 +
130.00 -
131.00 -
132.00 +

(126.5 is the local tracon frequency where the interference is
problematic for us, which is why I focused there. 127.0 might
be the strongest interference)

geographical location where interference seems strongest
(there's an antenna there, and when we were directly over
it interference stopped)
38 31 90
90 21 75

Can we figure out the frequency and maybe the station which
is causing the problem from the above info?

Ideas? Other tests? Things to check? Help! If we still
get the problem with the plane's entire electrical system off
and using a radio/antenna which is not connected to the plane,
is there ANYTHING we can do or must we just grit our teeth and
bear this?


Everything you say points to "intermodulation interference" -- where
multiple transmissions are hetrodyning against each other, and creating
a byproduct signal on the channel you're listening on. The bad news is
that locating the 'source' of the problem is *difficult*. In large part
because there isn't _a_ source. There are somewhere between 2 and "many"
'contributors' to the problem -- *all* of which have to be operating for
the problem to occur (that's why "sometimes it's there, sometimes it
isn't" -- sometimes one or more of the 'critical' contributor sources a
are -not- running.)

The fact that it happens with the plane electrical off tends to exonerate
_almost_all_ of the avionics from complicity.

There are, in essence, three possibilities remaining:

One: it is *possible*, albeit unlikely, that the hetrodyning is occurring
in an antenna/cabling/"front end" of one of the radios (even without
power applied), and being _re-radiated_ to be picked up by other radios,
including the hand-held.

The test, to eliminate this possibility, involves disconnecting antenna
cables (one at a time) from whatever they're plugged into, then (a)
shorting the center conductor to the shield, and (b) putting a 'dummy
load' (a 50 ohm resistor) across the radio connection. that radio
should now be totally 'deaf'. *IF* _it_ hears the interference, then
the 'intermod' is occurring at IF, and additional shielding around the
radio may help.

*IF* the problem 'goes away' with one of the antenna's shorted, you've
'localized' the problem, check for a corroded connection, cold-solder
joint, etc. consider replacing the entire assembly.

If, as is likely, nothing 'interesting' happens during the above testing,
It's time for _one_ additional test. using the _same_ hand-held, and
a *different* airplane, fly into the same area, at a time when you have
established that the problem is present. If the interference shows
up on the hand-held, you _have_ eliminated everything in the plane as
causative agent. It *is* 'inherent' in the locale If the radios in this
2nd plane are not picking it up, it is, for lack of a better term, a
"front end overload" problem in _your_ radios. Sensitivity to this
problem varies with the design of the radio, _and_ (although generally to
a minor degree) even from unit to unit within a given model line.

This is possibility #2, "out-of-band signal overload at the radio
front-end". A "High Q-Factor band-pass filter" installed on the
receive side of the radio can greatly reduce this problem. Depending
or radio design, this can range from 'reasonable' to 'outrageously
difficult and expensive' to implement.

The third possibility is that the hetrodyning, and re-radiation is
occurring somewhere _external_ to the airplane. e.g. _on_ some component
of that transmitting tower. In which case about the *only* 'solution'
is to treat it as a "DDT" problem -- since the problem occurs only when
you fly into that specific area, the solution is "Don't Do That!"
Don't fly into that area, and you won't have the problem.

Note: if this -is- the situation, then pretty much _everybody_ flying into
the vicinity should be experiencing the same problem, on a recurring basis.
Do other pilots report similar difficulties?


  #9  
Old November 4th 03, 05:16 AM
external usenet poster
 
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Default

Admin is correct as far as he goes.
This problem is also sometimes referred to as the "rusty bolt
problem". If you have a rusty bolt or rusty connection between two
metal parts on the antenna, the aircraft airframe , the transmitting
antenna and anywhere the field is strong enough the corrosion can act
as a diode and generate intermod products which is what you are most
likely hearing.

I would disconnect "ALL" of the antennas on the aircraft and use the
handheld while flying over this location. If this fixes the problem
connect one antenna at a time until the problem comes back. If the
signal is still present with all of the antennas disconnected it is
something on the ground or airframe corrosion. The next step is try
a different airplane with the same hand held. If it is still present
the problem is in or near the antenna farm on the ground.
Good luck
John


On Tue, 04 Nov 2003 02:18:14 GMT, ()
wrote:

In article ,
Snowbird wrote:


OK, we're still having our RF interference problem and
our avionics guy pleads 'stumped'. Meanwhile we're going
nuts whenever we need to get radar vectors for the ILS at
our local Class D or when we depart IFR to the SE.

Here is what we know

1) the problem is intermittant. occurs both at night and during day.
2) when it does occur, the problem occurs in a specific
area -- heading towards a local antenna farm
3) legitimate radio transmissions come through loud and clear
4) the interference isn't just random noise, but sometimes has
voices in it (like a radio or TV show)
5) we have disconnected the ELT from its antenna (but left
it turned off in the back seat of the plane) -- problem persists
6) marker beacons on, marker beacons off, nav radios different freqs,
nav radios off, no effect
7) swapped our KMA 20 audio panel for a loaner KMA 20 no difference
8) we have tried turning off the airplane's entire electrical
system and listening for interference on a handheld radio with its
own "stick" antenna. Problem persists (!!!!)
9) we have tried different frequencies while experiencing the
interference -- not exhaustively. here is a list (- means no
interference + means interference)

124.00 -
124.20 -
124.52 -
125.00 -
126.00 +
126.50 +
126.50 mb on, mb off, nav 111.9, nav 110.8, nav off
126.50 handheld w/ alt off, airplane electrical system off
127.00 +
127.10 -
127.25 -
127.27 +
127.30 -
127.50 +
127.97 -
128.00 -
129.00 +
130.00 -
131.00 -
132.00 +

(126.5 is the local tracon frequency where the interference is
problematic for us, which is why I focused there. 127.0 might
be the strongest interference)

geographical location where interference seems strongest
(there's an antenna there, and when we were directly over
it interference stopped)
38 31 90
90 21 75

Can we figure out the frequency and maybe the station which
is causing the problem from the above info?

Ideas? Other tests? Things to check? Help! If we still
get the problem with the plane's entire electrical system off
and using a radio/antenna which is not connected to the plane,
is there ANYTHING we can do or must we just grit our teeth and
bear this?


Everything you say points to "intermodulation interference" -- where
multiple transmissions are hetrodyning against each other, and creating
a byproduct signal on the channel you're listening on. The bad news is
that locating the 'source' of the problem is *difficult*. In large part
because there isn't _a_ source. There are somewhere between 2 and "many"
'contributors' to the problem -- *all* of which have to be operating for
the problem to occur (that's why "sometimes it's there, sometimes it
isn't" -- sometimes one or more of the 'critical' contributor sources a
are -not- running.)

The fact that it happens with the plane electrical off tends to exonerate
_almost_all_ of the avionics from complicity.

There are, in essence, three possibilities remaining:

One: it is *possible*, albeit unlikely, that the hetrodyning is occurring
in an antenna/cabling/"front end" of one of the radios (even without
power applied), and being _re-radiated_ to be picked up by other radios,
including the hand-held.

The test, to eliminate this possibility, involves disconnecting antenna
cables (one at a time) from whatever they're plugged into, then (a)
shorting the center conductor to the shield, and (b) putting a 'dummy
load' (a 50 ohm resistor) across the radio connection. that radio
should now be totally 'deaf'. *IF* _it_ hears the interference, then
the 'intermod' is occurring at IF, and additional shielding around the
radio may help.

*IF* the problem 'goes away' with one of the antenna's shorted, you've
'localized' the problem, check for a corroded connection, cold-solder
joint, etc. consider replacing the entire assembly.

If, as is likely, nothing 'interesting' happens during the above testing,
It's time for _one_ additional test. using the _same_ hand-held, and
a *different* airplane, fly into the same area, at a time when you have
established that the problem is present. If the interference shows
up on the hand-held, you _have_ eliminated everything in the plane as
causative agent. It *is* 'inherent' in the locale If the radios in this
2nd plane are not picking it up, it is, for lack of a better term, a
"front end overload" problem in _your_ radios. Sensitivity to this
problem varies with the design of the radio, _and_ (although generally to
a minor degree) even from unit to unit within a given model line.

This is possibility #2, "out-of-band signal overload at the radio
front-end". A "High Q-Factor band-pass filter" installed on the
receive side of the radio can greatly reduce this problem. Depending
or radio design, this can range from 'reasonable' to 'outrageously
difficult and expensive' to implement.

The third possibility is that the hetrodyning, and re-radiation is
occurring somewhere _external_ to the airplane. e.g. _on_ some component
of that transmitting tower. In which case about the *only* 'solution'
is to treat it as a "DDT" problem -- since the problem occurs only when
you fly into that specific area, the solution is "Don't Do That!"
Don't fly into that area, and you won't have the problem.

Note: if this -is- the situation, then pretty much _everybody_ flying into
the vicinity should be experiencing the same problem, on a recurring basis.
Do other pilots report similar difficulties?


 




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